Heat pumps and related devices have long been used for heating, air conditioning, refrigeration, and other heat transfer applications. Typically, such devices operate by transferring heat from a heat source, such as a source of waste heat or a conditioned space, to a liquid working fluid, thereby boiling the working fluid. The gaseous working fluid is then compressed, condensed to a liquid, and recycled to absorb additional heat from the conditioned space. The heat given up by the working fluid when it is condensed can be used for space heating, some other useful purpose, or can be rejected to the surroundings. While this cycle is widely used, drawbacks include the need for a compressor and the common use of chlorofluorocarbons as a working fluid. The compressor creates noise and includes moving parts that are subject to wear and failure. Chlorofluorocarbons are the subject of an international agreement that restricts their use in the future. In addition, prior art systems can limit the extent to which relatively low temperature waste heat (e.g., 35.degree. C. to 95.degree. C.) can be used to provide space heating.
As the industry strives to improve the reliability and performance of heat pumps and related devices such as air conditioners, reducing the noise produced by the compressor and eliminating the use of chlorofluorocarbons as working fluids are important goals. In achieving these goals, it also would be desirable to dispense with the compressor entirely. To date, however, these goals have been difficult to achieve. As a result, there remains a need for heat pumps that can be used to upgrade low grade waste heat, do not need a compressor, and do not use chlorofluorocarbons as a working fluid.